Alright, so I ended up digging through Archive of Our Own pretty hard last month when I couldn't find enough content elsewhere. The tagging system they have is the only reason I can reliably pull up stories that actually focus on Kana and Akane interacting, you know? You can search by the 'Arima Kana & Kurokawa Akane' tag for platonic stuff, or the '/' for romantic, and then filter by 'Oshi no Ko' and other fandoms. Wattpad is a mess for this kind of specific crossover, tags are all over the place. I found maybe two decent fics after scrolling through hundreds that just had their names in a giant character list.
Sometimes smaller, fandom-specific forums still have the best gems, though they're harder to find. I remember an old 'Oshi no Ko' forum that had a dedicated shipping sub-board, and someone wrote this really niche crossover with 'Chainsaw Man' where Kana and Akane were publicists dealing with devil hunters. The prose wasn't award-winning, but the character voices were spot-on. It's probably lost to time now.
Discord servers for the manga are weirdly the best for getting recommendations. Someone will drop a link to a fic on Pixiv or a Google Doc they wrote, and you get these raw, unfiltered stories that haven't been polished for a big platform's audience. The quality varies wildly, but the enthusiasm is always there.